This invention relates generally to a reproducing machine, and more particularly concerns the transfer of charged conductive particles from a latent image to a copy sheet.
Though there are various types of reproducing machines, the most commonly available are electrostatographic printing machines. In an electrostatographic printing machine, a latent image is recorded on a surface and rendered visible with particles. These particles may be transferred to a sheet of support material, in image configuration, or remain on the recording surface. In either case, the particles are permanently affixed to the sheet of support material or recording surface. In this manner, a copy of an original document is formed. Electrostatographic printing includes both electrophotographic and electrographic printing. Electrophotographic printing employs a light image of the original document to dissipate a charged photoconductive surface. This results in a latent image of the original document being recorded on the photoconductive surface. Electrographic printing does not employ a photoconductive member or a light image to create a latent image of the original document. In general, both of the foregoing processes employ heat-settable particles to develop the latent image. These are permanently affixed to the copy sheet by the application of heat hereto.
Typically, the developer material employed in an electrophotographic printing machine is a two-component mixture, i.e. a mixture of carrier granules and toner particles. Toner particles adhere triboelectrically to the carrier granules and, during the development process, are attracted from the carrier granules to the latent image. Typical toner particles used in a developer mix of this type have resistivities ranging from about 10.sup.14 to about 10.sup.17 ohm-cm. Generally, toner particles of this type are transferred from the latent image to a copy sheet by the application of a field across the photoconductive member-toner particles-copy sheet sandwich. In this way, the toner particles are attracted from the latent image to the copy sheet.
With the advent of single component developer materials, i.e. charged conductive particles, carrier granules are no longer required. However, the charged conductive particles employed in a single component system have low resistivities which range from about 10.sup.4 to about 10.sup.9 ohm-cm. These particles are also developed on the latent image recorded on the photoconductive member. However, when particles of this type are transferred from the latent image to the copy sheet, repulsion occurs. Repulsion is due to both the copy sheet and the charged conductive toner particles having relaxation times which are considerably shorter than the transfer time. This allows the charged conductive particles to exchange charge with the copy sheet, i.e. from positive charge to a negative charge, or vice versa.
Many different types of systems have been devised for improving both development and transfer of toner particles to the copy sheet. Exemplary of these are U.S. Pat. No. 3,882,822, issued to Sullivan, Jr. in 1975 and U.S. Pat. No. 3,676,533 issued to Gundlach, in 1971. Both of the foregoing patents appear to disclose a relatively narrow contact between the photoconductive surface and the copy sheet when compared to the development zone. U.S. Pat. No. 3,929,098 issued to Liebman in 1975 discloses the use of a corona generating device for spraying ions onto the backside of the copy sheet. The corona generating device appears to be located midway between the exit and entrance zones of the transfer station. In this way, toner particles are transferred from the photoconductive member to the copy sheet. Similarly, U.S. Pat. No. 3,881,927 issued to Fantuzzo in 1975 discloses a transfer station incorporating a wide transfer sheet contact zone with the photoconductive member in combination with a corona generating device disposed midway therebetween. U.S. Pat. No. 3,759,222 issued to Maksymiak et al in 1973 discloses the use of an alternating field to minimize charge buildup at dielectric interfaces. In addition, Xerox Disclosure Journal, Volume 1, Number 5, of May 1976, page 83, discloses a high frequency pulsed bias roll transfer system. The pulsing frequency is greater than the time constant of the copy sheet so that the applied transfer charge does not have sufficient time to dissipate. U.S. Pat. No. 3,147,679 issued to Schaffert in 1964 teaches the use of conductive rollers which urge the copy sheet into contact with the photoconductive drum. These rollers have at least peripheral conductive surface elements. The rollers are electrically insulated from the remainder of the transfer structure and have a polarity which aids and opposes charge transfer. However, none of the foregoing references appear to disclose a system wherein a single component developer material utilizing charged conductive particles is employed with the toner particles prevented from being repelled from the copy sheet.
Accordingly, it is a primary object of the present invention to improve electrophotographic printing to prevent the repulsion of charged conductive particles from the copy sheet.